I'm trying to understand one thing but I haven't understood it yet. So my problem is let's say we have a jet and I'm doing a transient analysis. I place the outlet at least 10 times far than the diameter of the jet. Or transient analysis of flow over a cylinder. When the created vortices by the jet or cylinder reach outlet not all the flow is going out. In that case should I use opening all the time? And also we do not know the inward flow direction angle - should I use entrainment option under opening BC?

There are many options. You could use an outlet and have walls created at the back flow sections. Or you could have several varieties of opening as you suggest.

They all will work. But the difference is that some of them will require the downstream boudary to be further away then others before you get acceptable accuracy. And a second major factor is they have different numerical stabilities (the simpler outlets are more stable than the more complex openings).

So the only universal answer is - do a sensitivity study and deteremine how far down stream the boundary condition you have chosen needs to be to get the accuracy you require. If you are lookign for the best option repeat this for a number of boundary types and you will find the best one for your case.